The YH\(_6\)  prediction was startling on two counts: predicted T\(_c\)  was 30% above the already remarkable SH\(_3\)  value of the previous year, and the required pressure was substantially lower. YH\(_6\)  contains some very strongly coupled H modes at comparatively low frequency (not far from instability), accounting for \(\lambda \approx 3\)  and high T\(_c\)  in spite of (in fact, related to) the significantly lowered phonon energy scale \(\omega _{log}\)  (the logarithmic mean frequency[1]}). However, anharmonicity and non-linear EP coupling can change predictions, especially when there are nearly unstable modes. These stoichiometries have not been reported in experimental studies as of 2022 (see below).
